Hydrogen peroxide has been used for bleaching wood pulp during paper manufacture. Sodium silicate is normally employed as a stabilizer to prevent early depletion of the active bleaching agent.
In the process of making the pulp, metal ions are picked up from the wood, the water and the machinery used to masticate the wood chips and pulp. While some of the metal ion content is lost in the deckering or dewatering step, it is sometimes advantageous to add a chelating agent. Of the commercially available chelating agents, the sodium salt of diethylenetriaminepentaacetic acid has been reported to be the most effective. This is found in an article "The Effect of DTPA on Reducing Peroxide Decomposition", D. R. Bambrick, TAPPI Journal, June 1985, pp. 96-100. The use of silicates in such systems, however, results in problems when insoluble silicates are deposited upon the fibers and the machinery employed. When deposited on the pulp fibers the result is a harsher feel of the paper. The fouling of equipment can cause down-time and shortened life of the equipment. Because of this, silicate-free systems have been suggested.
These silicate-free systems have been found to work well in the single stage hydrogen peroxide bleaching of Kraft pulps where the choice of stabilizer possibly influences the bleaching mechanism by changing the reaction pathway of hydrogen peroxide. In such systems, the addition of poly-(.alpha.-hydroxyacrylate) as a stabilizer also has been shown to improve pulp brightness. The use of this stabilizer is discussed in a paper "Hydrogen Peroxide Bleaching of Kraft Pulp and the Role of Stabilization of Hydrogen Peroxide," by G. Papageorges, et al given at the ESPRA Meeting in Maastricht, Netherlands, May, 1979. British Pat. No. 1,425,307 discloses a method for preparing this stabilizer.
In U.S. Pat. No. 3,860,391 the bleaching of cellulose fibers and mixtures thereof with synthetic fibers is accomplished by employing peroxide in a silicate-free system in the presence of an aliphatic hydroxy compound, an amino alkylenephosphonic acid compound and, alternatively, with the addition of a polyaminocarboxylic acid. Representative of the above are erythritol or pentaerythritol, ethylenediaminetetra(methylenephosphonic acid) or 1-hydroxypropane-1,1,3-triphosphonic acid and ethylenediaminetetraacetic acid or nitrilotriacetic acid, respectively.
Another patent, U.S. Pat. No. 4,238,282, describes a pulp bleaching system employing chlorine (not peroxide) which uses various chelating agents, including polyacrylic acid (mol. wt. &lt;2000), alkylene polyaminocarboxylic acids, and aminophosphonic acids and their salts.
Other more recent U.S. patents which employ such phosphonates as indicated above, but in a peroxide bleaching system, include U.S. Pat. No. 4,239,643 and its divisional U.S. Pat. No. 4,294,575.
While, as noted above, various combinations of chelating agents are useful in stabilizing peroxide bleaching systems, the presence of metal ions, e.g. iron, manganese and copper, provides a catalytic effect with respect to the decomposition of the peroxide and also tends to reduce the brightness of finished mechanical pulps. While the chelants might be expected to take care of minor amounts of the metal ions, the presence of significant amounts of magnesium and/or calcium ions which may be present in the wood pulp or water or both tends to overwhelm the ability of the chelants to complex the iron, manganese and copper ions present.
Certain combinations of the aminophosphonic acids together with polycarboxylic acids or polycarboxylic amides or a sulfonic acid derivative of a polyamide have been found to provide stabilization in the presence of significant amounts of magnesium and/or clacium ions and in the presence of small amounts of copper and the like metal ions which catalyze the peroxide decomposition. This stabilizer is disclosed in a copending application of one of the applicants in the present invention titled "Improved Stabilization of Peroxide Systems in the Presence of Alkaline Earth Metal Ions", Ser. No. 686,111, filed Dec. 24, 1984, now U.S. Pat. No. 4,614,646.
It has now been found that improved bleaching results by treating wood pulp with a polyaminocarboxylic acid prior to contacting the pulp with the stabilized aqueous peroxide solution referred to above.